1. Field of the Invention
This invention relates generally to cable television communication system distribution components. More particularly, the invention relates to a power inserter having dual power supply ports that can be internally configured to source power directionally or to use independent mains power supplies for dedicated directions in a CATV distribution system.
2. Description of the Prior Art
Cable television (CATV) services are provided to customers through a transmission network that typically includes a trunk system transporting the plurality of CATV services from a cable headend to smaller branches and then onto individual subscriber drops. The transmission medium for the branch and subscriber drops is usually coaxial cable. To provide for the transmission of the CATV services, the transmission network includes cable taps, cable splitters, line amplifiers and other miscellaneous equipment to distribute the CATV services.
A typical CATV cable plant is designed with unity gain from the headend to each subscriber terminal. Signal losses and gains throughout the trunk, branch lines and subscriber drops are noted and adjusted to maintain unity gain throughout. However, many of the aforementioned devices that distribute the cable services burden the system. As the CATV signal proceeds throughout the distribution system, the attenuation of the coaxial cable and the insertion loss of passive devices reduce the signal to less than unity thereby requiring periodic amplification. Line amplifiers are installed and adjusted accordingly to maintain unity gain.
Line amplifiers are usually suspended by the signal carrying coaxial cable support strand and are powered from the signal carrying coax cable. The usual method of distributing the mains power supply to each line amplifier is by impressing or inserting the alternating current on the coaxial cable via a CATV power inserter.
A power inserter consists of a low-pass filter designed for 60 cycle, 60 Vac and a band rejection filter for the RF signal components present on the coaxial cable. The power inserter is housed in a weather and RFI proof enclosure and is also suspended on the cable strand. At a line amplifier, filters separate the CATV RF signal from the impressed 60 Vac. The RF signal is amplified with the ac component providing the power source.
Power is usually supplied to a power inserter by a ferroresonant transformer located on a nearby telephone pole. The primary side of the ferroresonant transformer is connected to a single phase power supply from the local utility. Ferroresonant transformers provide isolation, regulation, and if necessary, reduction from the mains power supply.
The power inserter can provide power for several line amplifiers. Shown in FIG. 1 is a simplified branch 15 of a CATV distribution system. A prior art power inserter 17 is shown sourcing four line amplifiers 19 on either side of the power inserter 17. A ferroresonant transformer 21 is shown sourcing the power inserter 17. Both power and signal flow are bidirectional within the coaxial cable 23.
A schematic for the prior art power inserter 17 is shown in FIG. 2. The power inserter 17 has a single power jack or port 25, and first 27 and second 29 RF jacks or ports. The alternating or direct current 31 is filtered using a first 2-pole (2nd order) low-pass filter comprised of a first inductor 33 coupled to first 35 and second 37 parallel capacitors. The current flow for both coaxial cable directions is limited with first 39 and second 41 fuses to their respective first 27 and second 29 RF jacks. Power insertion may be directional in dependence upon one or both fuses being installed. The output from the first fuse 39 is low-pass filtered through a second 2-pole low-pass filter 47 and coupled to the signal conductor of the first RF jack 27. The output from the second fuse 41 is low-pass filtered through a third 2-pole low-pass filter 49 and coupled to the signal conductor of the second RF jack 29. To allow the RF signal to pass freely between the first 27 and second 29 RF jacks, a sixth capacitor 51 forms a high-pass filter between the RF terminals blocking the low frequency current thereby giving direction to power flow.
A variation of the single source power inserter 19 is shown in FIGS. 3 and 4. A dual alternating or direct current source power inserter 53 is shown having two isolated ac terminals (depending upon the configuration of an internal jumper 54) each sourcing a dedicated RF terminal. However, the prior art dual source power inserter 53 has limitations when configuring the power inserter 53 concerning isolation between RF terminals and power ports.
In order to increase the number of sourcing options available in one cable television power inserter, it is desirable to have a low cost device that can provide common or isolated directional power insertion or distribution.